Free, there are pretty much two sizes in hard drives, 2.5" for Laptops and the standard 3.5" for all other computers, Macs, PC, etc. Your iMac is not a laptop.
Before addressing make or model, you should set a minimum / maximum size and speed.
7200 rpm for you is both the minimum & maximum speed, it is perfect for appropriate FireWire speed or mounting internally onto your bus.
Faster won't really help you (10,000 rpm or 15,000 rpm), and
slower won't save you any money and you will instead be burdened with a "slow" drive. (5400 rpm and slower is a joke anymore)
80 GB is an acceptable minimum and should cost you about $80, or $1 per GB competitively. 120, 160, or 200 are also good, but the larger the drive, you will often find the slower it is going to react. A RAID of two 100 GB drives is twice as fast as one 200 GB drive, and would cost you about the same - as long as you have the room to mount both, and you don't. So, depending on how much money you want to spend - $80 to $160 for an 80 or 160 GB hard drive.
8 MB buffer or L2 cache has replaced 2 MB as the norm for hard drive memory, and larger means faster. So, avoid 2 MB and set 8 MB as a minimum.
9.5ms or lower access rate is acceptable (smaller number is better).
8.5ms is very good, do not tolerate 10ms or higher.
You are going to like how quietly an
Hitachi/IBM Travelstar (over 500 models to chose from)
or
SEAGATE Barracuda (over 355 models to chose from)
sounds. Good quality, solid feel, great reputation, excellent warranty & service. If you investigate the high-end technology of these 2 drive manufacturers, then you will understand why they appear to be head & tail above the rest. Their drives may cost a bit more, but you get lots more in quality.
After those drives, I like Western Digital, which has often been an OEM for Mac computer drives. Excellent Customer Assistance staff, very responsive.
Lastly, I'd settle for Maxtor or Toshiba, whose drives have improved reasonably well over the last several years.
Where to buy? Any of the Mac "warehouses" like CDW, OWC, MacMall, MacZone, etc. Another old, loyal Mac site is
http://www.SmallDog.com com. Two other very price competitive sites are
http://www.ZipZoomFly.com and
http://www.NewEgg.com
Always shop around for price AND shipping & handling combined cost, often /
Free 2nd Day. Here are some examples of very similar drives (check out full specs before buying, and compare):
SEAGATE Barracuda 7200.7 Plus 200 GB
Standard - INTERNAL - Ultra DMA/ATA-100 -
7200 Rpm Rotational Speed
Data Transfer Rate: 683 Mbps -
8.5ms Average Seek (msec)
8 MB L2 Cache
$145.00
Hitachi/IBM DeskStar 7K250 ATA/6 200GB
7200 RPM
8.5ms
8MB Buffer
3yr Hitachi/IBM Warranty.
$159.00
